Filter element using ferromagnetic material loading



July 29, 1969 DlNH-TUAN NGO FILTER ELEMENT USING FERROMAGNETIG MATERIAL LOADING Filed Dec. 22, 1966 LOAD D/[ L E C 772/6" /6 CE N TE I? CONDUCTOR /7 GROUND PLANE F E RROMAGNE T/ C SOURCE M MATERIAL D/E L E C TR/C COAX/AL (PRIOR ART) TH/N FILM THIN F/LM m x m m M lllilllltlll'I LLILIFIFFV III f r FREQUENCY FIG. 4

STATIC BIAS INVENTOR 0. N60 8) MW A TTORNEV United States Patent 3,458,837 FILTER ELEMENT USENG FERRQMAGNETIC MATERIAL LQADENG Birth-Titan Ngo, Somerset, N.J., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, Ni, a corporation of New York Filed Dec. 22, 1966, Ser. No. 603,314 Int. Cl. H011: 1/20 US. Cl. 333-73 12 (l'lairns ABSTRAQT @F THE DISCLQSURE A transmission line includes an element magnetically biased near ferromagnetic resonance to give the line a low pass filter characteristic with essentially flat response in the filter pass band. The magnetic bias is electrically controlled, and plural filter sections in tandem are biased by a multilevel magnetic field with a single electrical control.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to an electric signal filter employing biased magnetic means to fix the filter pass band and cutolf characteristics.

Description of prior art In prior art filters employing lumped circuit constants or employing, in the ultra high frequency range, a conductor shaped at specific different points to act as specific lumped impedance elements, it is known that the pass band attenuation-versns-frequency characteristic includes a ripple. Such ripple represents variations in attenuation of two or three decibels in some cases. Ripple of that magnitude is generally tolerable because it is substantially within the attenuation range considered to characterize the pass band of the filter. However, in some digital systems a cyclic attenuation variation such as the aforementioned ripple produces signal echoes which interfere with the pulse-type information representation employed in the system.

It is, therefore, one object of the invention to reduce ripple in the pass band of a filter attenuation-versusfrequency characteristic.

Furthermore, prior art filters, particularly those employed in higher frequency ranges, are not readily tunable. It is usually necessary to construct a substitute filter in order to shift the cutoff frequency.

It is, therefore, another object of the invention to facilitate the shifting of cutoff frequency in a filter element.

Statement of invention The aforementioned objects of the invention are realized in an illustrative embodiment in which a ferromagnetic material is arranged adjacent to a conductor of a transmission circuit and is magnetically biased to a level corresponding to ferromagnetic resonance at a predetermined frequency to give the circuit a filter-type attenuation-versus-frequency characteristic that is essentially fiat in the filter pass band.

It is one feature of the invention that electrical control is employed for the application of the magnetic bias to make the filter cutoff point electrically tunable.

It is another featuer that plural filter sections are advantageously combined in series and controlled by an electrically driven multilevel bias field.

The drawing The various features, objects and advantages of the invention may be more completely understood from a 3,453,837, Ice Patented July 29, 1969 consideration of the following detailed description in conjunction with the appended claims and the attached drawing in which:

FIG. 1 is a schematic diagram of a transmission circuit including the invention;

FIG. 1A is a cross-sectional view of a part of FIG. 1;

FIG. 2 is a diagram of a part of FIG. 1 indicating one set of various magnetic orientations therein; and

FIGS. 3 and 4 are diagrams illustrating electrical and magnetic characteristics to facilitate an understanding of the invention.

Detailed description In FIG. 1 a multifrequency signal source 10 produces a signal including frequency components within a predetermined band. Source 10 is coupled to a load 11 by means of coaxial cables 12 and 13 and an interposed two-conductor transmission circuit 16 which is adapted to operate as a low-pass filter. The transmission circuit 16 is illustrated in the form of a strip transmission line with a center conductor 17 and having symmetrically arranged on either side thereof under each section of a bias coil 21 a pair of dielectric material portions 18, a pair of ferromagnetic material portions 19, and a pair of ground conductor portions 20. FIG. 1A shows a cross section of the mentioned symmetrical arrangement, as found under any section of coil 21, and taken perpendicular to the propagation direction in conductor 17. However, in FIG. 1 only the ferromagnetic material portions 19 under coil section 21a are indicated by reference character.

Each dielectric portion 18 is advantageously a low loss type of glass slab with Permalloy thin films, for portions 19, deposited on the surface thereof facing the respective ground plane conductors 20. Of course, the film can be deposited on a conductor, or different dielectrics can be employed. As is known in the art, such films generally comprise an alloy of metals such as nickel and iron. One such alloy includes approximately 81 percent nickel and 19 percent iron. The magnetic thin film portions 19, one of which is shown in FIG. 2 with the same orientation as in FIG. 1, are anisotropic thin films. As shown in FIG. 2 each film has the easy magnetization axis thereof in the plane of the film and oriented in the direction of signal propagation through the transmission circuit 16 from the input cable 12 to the output cable 13 as can be seen by comparing FIGS. 1 and 2 of the drawing. The easy axis in FIG. 2 is indicated by an arrow h and the film exhibits bistable remanent magnetic flux conditions of opposite polarity in the direction of that arrow. An arrow h in FIG. 2 indicates the corresponding orientation of the hard axis of the film and is in the plane of the film and orthogonal to the direction of propagation in circuit 16. The arrow H in FIG. 2 indicates the orientation of a static magnetic bias which is applied to each symmetrical pair of the film portions 19 by its associated section of coil 21 in FIG. 1.

With hard and easy axes oriented as shown in FIG. 2, the lowest cutoff frequency of the filter depends upon the anisotropy field H of a particular film. For example, using present films the cutoff frequency can be taken down to about 600 megahertz for a film with H of about 4 oersteds. It has also been found that with hard and easy axis orientations interchanged the theoretical lowest cutoff frequency is zero, but for any given cutoff frequency an additional bias of about 2H is necessary for the lastmentioned orientation as compared to that shown in FIG. 2.

Coil 21 is energized by a battery 22 which supplies current thereto through a rheostat 23 for adjusting the current level in the coil 21 in order to change the magnetic bias and thereby tune the filter for different cutoff frequencies as will be subsequently discussed in greater detail. Thus, the magnetic film portions 19 are subjected to a magnetic bias which is oriented longitudinally along the direction of propagation of the multifrequency signal from source 10. The transmission circuit 16 is essentially a two-conductor transmission circuit since both of the ground plane portions 20 are connected to ground. Accordingly, such circuit favors the TEM mode of signal transmission. In such mode the magnetic component of the oscillating signal from source is, at any point along circuit 16, in a plane which is perpendicular to the direction of propagation. A magnetic field through the film portions 19 is provided by such magnetic component as indicated by the arrow H in FIG. 2. Both of the film portions 19 of a pair in FIG. 1 have magnetic orientations as discussed in connection with FIG. 2, and the oscillating magnetic field H links both films of each pair in the circuit 16.

Although a strip line is shown in FIG. 1, the invention is also applicable to coaxial transmission circuits wherein, for example, the magnetic thin film is interposed between the dielectric material and the inner surface of the outer conductor of the coaxial circuit.

Coil 21 is shown in plural sections 21a through 21d for a reason to be subsequently discussed, but initially the operation of circuit 16 parts enclosed by only the coil section 21a are considered. FIG. 3 includes a solid-line attenuation-versos-frequency characteristic for the part of transmission circuit 16 enclosed by coil section 21a. In order to produce such characteristic, battery 22 is caused to supply sufficient current to the coil 21 to bias the film portions 19 under coil section 21a to ferromagnetic resonance corresponding to a frequency in the multifrequency signal from source 10. The bias field produced within coil section 21a produces in the film portions 19 therein a substantially uniform bias level corresponding to a ferromagnetic resonant state for the frequency f Uniformity of bias has reference to any film cross section, and it is not necessary that each cross section along circuit 16 have the same bias level. Under these conditions the circuit 16 functions as a low pass filter with a cutoff frequency A, below the frequency f In the present illustration the cutofit frequency is, for convenience, defined as the increased attenuation point at which transmission is down three decibels. At the frequency f; transmission in the circuit 16 is approximately three decibels down from the level prevailing throughout the remainder of the pass band of interest, which includes the uniformly spaced frequency components f through 1, in the multifrequency signal from source 10.

It can be seen in FIG. 3 that the attenuation characteristic for a thin film loaded circuit is essentially uniform in the band including the frequencies f through f Such response results from the presence of a static bias, generated by coil 21, corresponding to resonance at the frequency f,. A dashed curve in FIG. 3 is superimposed upon the solid line curve to illustrate a typical attenuation-versusfrequency characteristic for the mentioned prior art coaxial filter which is designed to have essentially the same pass band and cutoff frequency as the transmission circuit 16 of FIG. 1. However, the coaxial filter characteristic in FIG. 3 includes a cyclic ripple having a magnitude of two to three decibels with respect to the fiat transmission illustrated for the circuit 16 of FIG. 1. Such ripple is objectionable because it can have the effect of generating echogs in certain transmission circuits as hereinbefore note A single filter section of the type hereinbefore described has at frequencies above the resonant frequency f, thereof a substantially flat response similar to that in the mentioned low pass band. However, above the frequency f,- the attenuation is greater than below 1, because the filter impedance is different and is not matched wilth the characteristic impedance of cables 12 and 13 whereas it is advantageously matched in the pass band. Greater inser- 4 tion loss occurs above f but it is not nearly as much as occurs near h. Similarly, if filter impedance is advantageously matched a low standing wave ratio characterizes the filter below the cutoff frequency, but unmatched impedance and high standing wave ratio appear above that frequency.

In order to realize a broader stop band above the desired cutoff frequency, plural filter sections in electrical tandem are employed; and this is the full format shown in FIG. 1. FIG. 3 includes a dash-dot curve representing the attenuation-versus-frequency characteristic of a multisection transmission line filter of the film-loaded type hereinbefore described.

The pairs of magnetic thin film portions 19 of FIG. 1 are separated into discrete segments with individual bias coil 21 sections coupled respectively thereto. Alternatively, however, the film may include similar sections in a single film segment which is then subjected to a corresponding multilevel magnetic bias field. The coil 21 includes the sections 21a, 21b, 21c, and 21d which are connected in series with one another and with the rheostat 23 across the terminals of the battery 22. Each coil section has a different number of turns for biasing its associated film portions 19 for ferromagnetic resonance at different frequencies f and f and f,'. Thus, if section 21d biases its films to resonance at f,, the sections 210, 21b, and 21a, with increasing numbers of turns, bias their respective film pairs to resonance at the higher frequencies f f and 11', respectively.

It is known in the art that thin film can be used to load two-conductor circuits for producing a constant phase variable attenuator. However, such an attenuator is usually operated near resonance in a single-frequency application. Consequently, moderate changes in bias produce substantial attenuation changes and substantial phase changes. The latter changes must be compensated to get the mentioned constant phase result. No phase compensation is required for a filter section as shown in FIG. 1 because the pass band of interest is apart from the resonant point of the films in a frequency sense. In that pass band impedance is substantially constant, and there is no extraordinary phase shift beyond that normally found in a strip transmission line of the same make-up but lacking film loading.

In considering the construction of the transmission circuit filter of the type illustrated in FIG. 1, the designer must select a center conductor 17 with dimensions which are suitable to the signal frequency band of interest as is known for such conductors in strip transmission lines. A similar selection is made for the ground plane portions 20. The latter portions are illustrated for convenience as having the same width as the central conductor 17, but it is to be understood that in many applications it is advantageous for the ground plane conductors to be significantly wider than the center conductor, as is known in the art. A convenient dielectric material and thickness are also selected by the designer; and, for example, glass plates with Permalloy film deposited on one side thereof are advantageously employed. The width of the magnetic film portions 19 is advantageously selected to be the same as the width of the center conductor 17.

The film thickness for portions 19 must be less than the skin depth of the highest frequency in the desired pass band in order to excite a uniform ferromagnetic resonant mode, as hereinbefore described, throughout a cross section of the film. However, the film should be thin enough so that the magnetization vectors rotate essentially in a plane parallel to the film plane. Where the cutoff frequency of the filter is to be tunable, it is necessary that the film thickness be selected to be less than skin depth of the highest anticipated cutoff frequency in the tuning range. It has been found that if a substantially uniform ferromagnetic resonant mode is not produced throughout the film portions 19, the circuit will have substantially higher insertion loss in the pass band. It also causes a marked reduction in the sharpness of the attenuation-versus-frequency characteristic at the cutoff frequency. It has been further found that the use of films of less than 500 A. requires film lengths that are inconvenient in order to get enough ferromagnetic material volume to produce the desired attenuation levels outside the pass band.

The remaining parameters of the transmission circuit 16 for a single filter section, e.g., that under coil section 21a, are film length and bias field strength. The film length can be readily computed by techniques which are known in the art. Thus, for example, the designer determines the permeability of the particular film as a function of static bias field by employing the Landau-Lifshitz equation which is known in the art. In this connection, see page 6 in subsection 1A of doctoral thesis of D. Ngo entitled A Strip Line With Variable Magnetic Film Discontinuity, dated 1962, and on file in the library of Iowa State University of Science and Technology in Ames, Iowa. Using that permeability the designer then solves the Maxwell equations for a strip line having a ground plane with adjacent Permalloy films in order to get the propagation constant of the line. Such constant is expressed in terms of attenuation per unit of length. Knowing the total attenuation which is desired at the cutoff frequency, the length of the film can be determined from the propagation constant.

At this point it remains only to determine the appropriate static magnetic bias field which must be applied, i.e., to determine at what frequency the magnetic film portions 19 should be capable of resonating in order to produce the aforementioned desired attenution at the cutoff frequency and with essentially fiat response in the pass band. It has been found that this latter information can be determined by plotting a family of propagation constant-versusstatic bias characteristics for different frequencies of interest, as shown in FIG. 4. The characteristic for each frequency in FIG. 4 approaches the horizontal axis asymptotically as static bias is increased beyond the bias corresponding to ferromagnetic resonance for such frequency. The characteristics are shown, at least in part, in FIG. 4 for the frequencies f through f and f,. With this family of characteristics, the designer then finds the characteristic for his desired cutoff frequency and looks in the static bias range above resonance for that characteristic for a common bias level at which the characteristics for uniformly spaced frequencies across the pass band of interest are also uniformly spaced at the common static bias level. This point is designated as the bias H in FIG. 4 and corresponds approximately to the ferromagnetic resonant bias at a signal frequency f, which was previously not specifically identified. When the film portions 19 of a filter section are uniformly subjected to the static bias level H indicated in FIG. 4, they cause the transmission circuit 16 to produce the solid-line attenuation-versus-frequency characteristic illustrated in FIG. 3.

In one embodiment that was actually constructed and operated for a single section filter with a theoretical pass band down to zero frequency, the principal parameters were as follows:

Cutoff frequency-nominal value in tuning range 1 gigahertz. Film thickness 1.0x meter. Film width 0.318 centimeter. Film length 2.5 centimeters. Dielectric type Corning Microscope slide glass.

Dielectric thickness 1.513 10 meter. Conductor 17-width inch. Conductor 17thickness 0.004 inch.

Such filter had a characteristic impedance in the pass band of about 10 ohms. A 50-ohrn filter can be made by using film of four times the indicated length and dielectric of five times the.indicated thickness. The latter type filter can, of course, be coupled to widely-used SO-ohm cable without impedance transformation which often has a band limiting effect.

For a transmission circuit with plural filter sections to produce a characteristic such as the dash-dot characteristic in FIG. 3, the previously outlined design procedures are followed for each section except that the resonant frequency and static bias level for each additional section after the first are not selected from FIG. 4. The resonant frequency and corresponding bias for each successive section are selected to produce in the stop band above 1, a desired minimum average attenuation with respect to the pass band attenuation. The extent of the stop band of high attenuation and the magnitude of ripple in that part of the characteristic are functions of the number of sections employed and the spread among resonant frequencies as will be readily appreciated by those skilled in the art.

Although the invention has been described in connection with a particular embodiment thereof, it is to be understood that additional embodiments and modifications which will be obvious to those skilled in the art are included within the spirit and scope of the invention.

What is claimed is:

1. In a filter comprising input means for receiving a multifrequency signal and an output circuit, the improvement comprising a transmission line coupling components of said signal below a predetermined frequency from said input means to said output circuit, said line having a first conductor, a ground plane conductor, and dielectric material between said first conductor and the ground plane conductor,

an electrically conductive ferromagnetic member between one of said conductors and an adjacent portion of said dielectric material, and

means applying to said member a magnetic field perpendicular to the magnetic component of said signal and of sufficient magnitude to bias said member to a level corresponding to a ferromagnetic resonance condition at a second frequency above said predetermined frequency to provide a substantially uniform impedance to a predetermined hand of frequencies of said signal below said predetermined frequency.

2. The filter in accordance with claim 1 in which said ferromagnetic member is a magnetic thin film having anisotropic magnetic characteristics with hard and easy magnetization axes in the plane of said film and orthogonal to one another, one of said axes being parallel to the direction of signal propagation in said line.

3. The filter in accordance with claim 1 in which said ferromagnetic member is a magnetic thin film having anisotropic magnetic characteristics with an easy magnetization axis in the plane of said film and orthogonal to the direction of signal propagation in said line and a hard magnetization axis in the plane of said film and parallel to said direction of propagation.

4. The filter in accordance with claim 3 in which said ferromagnetic thin film is adjacent to said ground plane conductor.

5. The filter in accordance with claim 1 in which said ferromagnetic member is biased substantially uniformly throughout any given cross section thereof.

6. The filter in accordance with claim 1 in which said transmission line is a strip transmission line having ground plane conductors and dielectric material portions symmetrically disposed on either side of said first conductor, and

said ferromagnetic member includes portions between each of said ground conductors and its corresponding dielectric material portion.

7. The filter in accordance with claim 1 in which said ferromagnetic member separates its adjacent conductor and dielectric material by a distance corresponding to the thickness of such member, said thickness being less than the skin depth of said predetermined frequency component of said signal.

7 8 8. The filter in accordance with claim 7 in which material between said first conductor and the ground said ferromagnetic member has a length proportioned plane conductor,

with respect to the propagation constant of said a ferromagnetic member between one of said conductransmission line to produce a predetermined attenutors and an adjacent portion of said dielectric mateation level, with respect to signal transmission level rial, said member including a plurality of sections in in said predetermined band, at said predetermin d successive tandem parts of said transmission line,

and

means applying to said member a magnetic field perpendicular to the magnetic component of said signal and of sufiicient magnitude to bias said member to a level corresponding to a ferromagnetic resonance condition at a second frequency above said predetermined frequency to provide a substantially unifrequency. 9. The filter in accordance with claim 1 in which propagation constant-versus-magnetic bias field characteristics for predetermined frequencies of said sig- 10 nal are, at a bias level corresponding to said second frequency, substantially uniformly spaced at uniformly separated frequencies in said predetermined form impedance to a predetermined band f f band bel w Sa1d pre e r eq p quencies of said signal below said predetermined 10. The filter in accordance with claim 1 1n which frequency, said field applying means including a corsaid magnetic field applying means comprise a coil responding plurality of field applying sections assowrapped around a portion of said transmission line ciated with respective ones of said transmission line including said ferromagnetic member, and tandem parts for biasing said ferromagnetic member means applying dilferent selectable cur t l l t sections therein to different levels each correspond- Said coiL ing to a ferromagnetic resonance condition at a dif- IL The filter in accordance with claim 1 in which ferent frequency above said predetermined frequency.

said means coupling said transmission line between said input means and said output circuit consists of References Cited direct wire connections. UNITED STATES PATENTS 12, In a filter comprising input means for receiving a 3,072,869 1/ 1963 Seidel 333-73 X multifrequency signal and an output circuit, the improvement comprising HERMAN KARL SAALBACH, Primary Examiner a transmission line coupling components of said signal VEZEAU, Assistant Examiner below a predetermined frequency from said input means to said output circuit, said line having a first US. Cl. X.R. conductor, a ground plane conductor, and dielectric 33324 

